Lithium ion secondary batteries have high energy density and excellent charge/discharge cycle characteristics, and are thus widely used for a power supply for compact mobile devices such as cellular phones and laptop computers. In addition, the recent increasing environmental considerations and growing consciousness of energy saving have been promoting a demand for large batteries having a large capacity and a long life in the fields of electric vehicles, hybrid electric vehicles, power storage, etc.
In general, a lithium ion secondary battery primarily consists of: a negative electrode including a negative electrode active material of a carbon material capable of intercalating and deintercalating a lithium ion; a positive electrode including a positive electrode active material of a lithium composite oxide capable of intercalating and deintercalating a lithium ion; a separator separating the negative electrode and the positive electrode; and a non-aqueous electrolytic solution prepared by dissolving a lithium salt in a non-aqueous solvent.
Amorphous carbon or graphite is used for the carbon material used as the negative electrode active material, and graphite is typically used particularly in an application which requires a high energy density.
For example, Patent Literature 1 describes a carbon material for a negative electrode for a non-aqueous electrolyte secondary battery, the carbon material including a mixture of an artificial graphite particle and a natural graphite particle at 50:50 to 80:20 (mass ratio), wherein the artificial graphite particle has an interplanar spacing for the (002) plane, d002, of 0.3354 to 0.3360 nm in the X-ray diffraction pattern, and an average aspect ratio of 1 to 5; the natural graphite particle has an interplanar spacing for the (002) plane, d002, of 0.3354 to 0.3357 nm in the X-ray diffraction pattern, a median diameter (D50) of 10 to 25 μm, and relations among the D50, the diameter at 10 cumulative % (D10), and the diameter at 90 cumulative % (D90), specifically, D90/D50 and D50/D10 are each 1.6 or smaller. The literature states that an object of the invention is to provide a non-aqueous electrolyte secondary battery excellent in charging/loading characteristics in a low-temperature environment by using such a carbon material.
Patent Literature 2 describes a negative electrode for a non-aqueous electrolyte secondary battery, the negative electrode including a first carbon capable of electrochemically intercalating and deintercalating a lithium ion; and a second carbon capable of electrochemically intercalating and deintercalating a lithium ion or substantially incapable of intercalating a lithium ion, wherein an aggregate of the second carbon particle is primarily localized in an empty space among a plurality of particles of the first carbon, and the average particle diameter of the second carbon is 15% or less of the average particle diameter of the first carbon. The literature states that an object of the invention is to provide a non-aqueous electrolyte secondary battery with such a negative electrode in which the peeling of a mixture layer caused by charge/discharge cycles can be prevented and which provides a high capacity.
Patent Literature 3 describes a negative electrode material for a non-aqueous electrolytic solution secondary battery, the negative electrode material including a graphite particle (A) and a carbon material (B), wherein the graphite particle (A) has an interplanar spacing for the 002 plane (d002) of 3.37 Å (0.337 nm) or smaller as measured with a wide angle X-ray diffraction method, and an average roundness of 0.9 or higher; the carbon material (B) has an interplanar spacing for the 002 plane (d002) of 3.37 Å (0.337 nm) or smaller, a Raman R value (peak strength around 1360 cm−1/peak strength around 1580 cm−1) of 0.18 to 0.7 in the Raman spectrum with an argon ion laser, an aspect ratio of 4 or larger, and an average particle diameter (d50) of 2 to 12 μm; and the mass fraction of the carbon material (B) to the total amount of the graphite particle (A) and the carbon material (B) is 0.5 to 15% by mass. The literature states that a non-aqueous electrolytic solution secondary battery with such a negative electrode material exhibits low irreversible capacity and excellent properties in terms of charge/discharge efficiency.